Animal allergens are major components of house and animal laboratory dust.

Allergen Exposure

Geographical distributionNative to Asia, House Mice are now ubiquitous. They exist in all climates and are routinely found both indoors and out. Their constant gnawing damages buildings, furniture and equipment. Mice carry diseases such as salmonella and leptospirosis. They are everyday pests because of their consumption of foodstuffs, and also because their continual dribble of urine and their feces cause contamination. Mice breed more prolifically than Rats, and spread faster, being smaller (which makes concealment easier) and more migratory, but are more easily controlled by poison, traps and predators. They are generally not as troublesome as Rats. In cultivated fields they may be beneficial, eating weed seeds and insects.

EnvironmentAgile and having a varied diet, Mice are found in every kind of building. They are a particular problem in poultry units, Pig housing, grain stores, warehouses, shops, and hospitals. They often migrate from cultivated fields into buildings after harvest. Where conditions permit, Mice may be found in meadows, along watercourses, and in other places where vegetation is dense enough to afford concealment, but they are not nearly as common in undisturbed or natural habitats.

Especially because of the numbers of Mice used in laboratories, allergy to Mice is an important occupational health problem.

Unexpected exposureLike Rats, Mice interact unseen with humans, through mainly nocturnal foraging, which leaves behind urine, feces, saliva and skin flakes on many surfaces, especially those used for the preparation of food.

AllergensMouse allergens were identified over 2 decades ago. Major allergens were found in Mouse skin, serum, and urine: a 67 kDa protein, identical to Mouse albumin, and an approx. 17 kDa protein. Some individuals were sensitised predominantly to the large allergen, some to the smaller allergen, and one group of patients reacted to both allergens (1).

The concentration of the major allergens from Mouse, including Mouse serum albumin (MSA) and Mouse urinary protein (MUP) complex, vary in urine, serum, and pelts of Mice (2).

To date, a number of allergens have been characterised.

Mus m 1is a major allergen and a prealbumin. This 19 kDa protein is found in hair, dander and urine. This allergen is a lipocalin-odorant binding protein (3, 4) and was formerly known as MUP (major urinary protein) and also known as MAI and Ag1 (5-11) . Mus m 1 is produced in liver cells, circulates in the bloodstream, and is cleared by the kidneys. Males produce approximately 4 times more of this allergen than females. The allergen is low in serum (6, 12).

Mus m 2, a 16 kDa glycoprotein, is found in hair and dander.

Hair and epithelial fragments also carry allergenic molecules, which are primarily derived from urine and saliva. Most of the allergenic components of urine and saliva have also been detected in the fur extract. Significant concentrations of airborne rodent allergens have been measured in both laboratories and apartments in inner cities (13-15).

Mouse epithelium may be present in dust, and these Mouse allergens are carried mainly on particles of 6-18 microns. Allergen levels have been shown to correlate well with the number of animals present in the room and the degree of worker activity during sampling (16). The higher the number of animals in a room, the higher the allergen concentrations, and higher concentrations were also associated with cleaning activities. The highest personal exposure levels occurred when contaminated bedding and high numbers of conscious animals were handled. The highest airborne Mouse allergen levels have been reported to occur during manual emptying of cages, during changing of cages on an unventilated table and during handling of male animals on an unventilated table (17, 18). The proportion of time spent on these tasks determined the degree of allergen exposure to a large extent.

The concentration and type of Mouse allergen varies between locations and within the same location. Mouse pelt extract allergenic activity may be detected in rooms away from Mouse-care rooms, whereas Mouse urine allergenic activity may be found only in the Mouse-care room. In a study, airborne allergen content ranged from 1.8 to 825 ng/m3 and varied according to both the number of Mice and the amount of work activity in the rooms (2).

Mouse allergens are also very prevalent in ordinary homes. Of inner-city homes in Baltimore and Cleveland, USA, 95% had detectable Mouse allergen (Mus m 1) in at least one room, with the highest levels found in kitchens (kitchen: range, 0-618 microg/g; median, 1.60 microg/g; bedroom: range, 0-294 microg/g; median, 0.52 microg/g; television-living room: range, 0-203 microg/g; median, 0. 57 microg/g). By city, 100% of the kitchens in Baltimore had detectable Mouse allergen, with a lower percentage (74%) in Cleveland. Mouse allergen levels correlated according to room. Furthermore, 49% of the homes had reported problems with Mice within the previous year, and 29% of the homes had evidence of Mice in one or more rooms on home inspection; these homes had higher levels of Mouse allergen. Higher allergen levels were also associated with evidence of cockroach infestation in any room (19).

Potential Cross-Reactivity

Practically all respiratory animal allergens, including Mouse, characterised at the molecular level belong to the lipocalin family of proteins. Examples are the major allergens of Horse, Cow, Dog, Mouse and Cockroach as well as beta-lactoglobulin of Cow's milk (4). A certain degree of cross-reactivity is thus possible.

Clinical Experience

IgE mediated reactionsMouse allergens found in dust, urine, epithelium and saliva are a frequent cause of asthma, allergic rhinitis and allergic conjunctivitis, mainly in laboratory workers but also in ordinary individuals (2, 20-22).

Various studies have examined the prevalence of allergic disease in the work place to Mouse. Initial studies reported that about 20% of the exposed workers have symptoms of allergy to Mice (23). In a study evaluating the risk of laboratory animal allergy among research staff working in laboratories separate from the animal confinement area, 20% of the subjects had serum specific IgE >0.35 kU/l to Rat urinary allergens and/or Mouse urinary allergens, and 32% had experienced animal work-related symptoms, although 90% of aeroallergen samples from the laboratories in question were below the detection limit. More than 4 years of exposure significantly increased laboratory animal sensitisation and symptoms. Working mainly with male rodents resulted in increased risk for sensitisation and for symptoms (24).

Hollander et. al. demonstrated that the prevalence rates of allergy symptoms caused by working with Rats and Mice were 19% and 10%, respectively (25).

A large epidemiological study of 5000 laboratory workers reported symptoms in 26% exposed to Mice, 25% to Rats, 31% to Guinea Pigs, 30% to Rabbits, 26% to Hamsters, 25% to Dogs, 30% to Cats and 24% to Monkeys (26). Allergic rhinoconjunctivitis with nasal congestion, rhinorrhoea, sneezing and itchy, watery eyes can occur in up to 80% of symptomatic workers (9).

Although Mouse allergen is known to cause occupational asthma in laboratory workers, its potential significance in home environments has been underplayed. Through skin-specific IgE tests, 89 (18%) of 499 inner-city children were shown to be sensitised to Mouse. Children whose homes had Mouse allergen levels above 1.60 microg/g in the kitchen had a significantly higher rate of Mouse sensitisation than those with levels below (23% vs 11%). Atopy was also significantly related to Mouse sensitisation, with 40% of those with more than 4 positive skin-specific IgE responses having Mouse sensitivity, compared with 4% of those with no other positive skin-specific IgE responses (27).

Two hundred and sixty-three United Arab Emirates nationals with a respiratory disease suspected of being of allergic origin were submitted to skin- and serum-specific IgE measurement. Of these individuals, 8.3% were sensitised to Cat fur, 4.9% to Goat hair, and 0.7% to Rat hair and Mouse hair (28).

Importantly, children of parents exposed to Mice, Rats and Hamsters in an occupational setting, e.g., a laboratory, were shown to be more likely to have allergic symptoms, and to have significantly more positive skin-prick tests against allergens from the hair of laboratory animals, compared to children of non-exposed parents (29).

Occupational contact urticaria due to Mouse hair has been reported (30).

Other reactionsContact urticaria in laboratory technicians has been reported (31).